1. Technical Field
The present invention generally relates to a turbocharger and more particularly to an improvement of a structure for feeding oil to a bearing unit supporting a rotating shaft in a turbocharger housing and an improvement of a structure for supporting a semi-float bearing of the bearing unit in the turbocharger housing.
2. Background Art
Generally, a turbocharger housing is comprised of three major parts, i.e., a turbine housing, a compressor housing and a bearing housing. A typical example of such a turbocharger housing is illustrated in FIG. 6 of the accompanying drawings. As depicted, the turbocharger housing y includes a turbine housing b for housing a turbine rotor a, a compressor housing d for housing a compressor impeller c and a bearing housing g for housing a set of bearings f supporting a rotating shaft e. The shaft e connects the turbine rotor a with the compressor impeller c.
The turbine housing b and the bearing housing g are fixedly aligned by a pin h provided at their interface k. An outer periphery of the interface k is firmly held by a clamp i. The bearing housing g and the compressor housing d are joined with each other by a bolt j.
The bearing set f includes a plurality of journal bearings f1 and a thrust bearing f2. The journal bearings f1 are rotatably fitted in a shaft bore formed in the bearing housing g. The shaft e extends through the shaft bore. The thrust bearing unit f2 is provided at the right end of the shaft bore. Each of the journal bearings f1 is a cylindrically-shaped, full-float bearing which is rotatable in the rotating shaft bore of the bearing housing g. The journal bearings f1 support a radial force. The thrust bearing f2 includes a first ring-shaped bearing member fa secured to the rotating shaft e and a second bearing member fb engaged over the first bearing member fa and fixed to the bearing housing g. The thrust bearing f2 supports a thrust force.
In the illustrated turbocharger housing y, the journal bearings f1 and the thrust bearing f2 are assembled in the bearing housing g such that the number of necessary parts is large and the assembly process is troublesome and time consuming. In addition, a degree of freedom in designing oil feeding and discharging passages and spaces for the bearings f1 and f2 is quite limited due to needed accuracy and the shape of the core used in a molding process, as well as structure and locations of the bearings f1 and f2.
Particularly in case of a small turbocharger, it is impossible to design oil passages and spaces which can optimally control flow of a lubrication oil in the bearing housing g.
In order to simplify the assembling process, to reduce the number of necessary parts and to provide a controlled flow of oil in the turbocharger housing y, it is proposed to separate the bearing housing from the turbocharger housing y and to employ a semi-float bearing unit (metal bearing) which functions as both the journal and thrust bearings instead of the full float bearing. This simplifies the assembling process, reduces the number of necessary parts and provides a larger freedom in designing the oil passages and spaces in the turbocharger housing. Specifically, an independent bearing box is prepared separately, and a semi-float metal bearing is provided in the bearing box in such a manner that it can move in a radial direction and is fixed in axial and radial directions relative to the bearing box. This one-piece, semi-float bearing supports the rotating shaft in a full float manner and bears a thrust load.
However, this semi-float bearing has the following drawbacks. The semi-float bearing supports thrust loads at its ends so that the oil flow is more unstable than conventional structures on both the turbine and compressor sides. Accordingly, the lubrication oil tends to flow to sealing members provided between the turbine rotor and the rotating shaft (or bearing box) and between the compressor impeller and the rotating shaft (or bearing box). Thus, the desired oil discharge flow is not realized.
Furthermore, the semi-float bearing is generally secured onto the turbocharger housing by a thrust pin. However, the semi-float bearing is made from a soft material, such as copper, which provides an excellent bearing property, so that the semi-float bearing is easy to wear at the point of linear contact between the thrust pin and the semi-float bearing.
Another known turbocharger housing is disclosed in, for example, Japanese Patent Application Laid-Open Publications Nos. 7-139363 and 7-150962.